This invention pertains generally to methods of removing hydrogen and its isotopes from various atmospheres within enclosed spaces and particularly to the use of novel organic polymer formulations combined with hydrogenation catalysts, as absorbers of hydrogen and its isotopes.
In many applications the presence of hydrogen and its isotopes, arising from various chemical and electrochemical corrosion reactions, can be detrimental. Hydrogen can evolve from corrosion by atmospheric gases; corrosion generated by stray electric currents; from electronic devices, which can include batteries, operating in normal or abnormal condition; corrosion in heat exchangers; and from leaky hydrogen piping. Besides presenting a significant fire and/or explosion hazard, particularly in sealed components, hydrogen has a high thermal conductivity for a gas and can foul an insulating vacuum more rapidly than any other gas. Hydrogen can also react with many substrate materials significantly altering their physical and chemical properties. Mechanical and electrical safety devices, such as pressure relief valves, can be expensive, unreliable, and, particularly for small devices, not always practical. In the case of heat exchangers, the accumulation of hydrogen within the heat exchanger causes the thermal insulating and transfer properties of the heat exchanger to degrade over time.
It has long been known that hydrogen absorbing materials, known as getters, can be used to counteract hydrogen accumulation. Ayers, et al., discuss the use of active metals such as zirconium or titanium, and alloys thereof, in U.S. Pat. No. 4,512,721. These metals are capable of maintaining low hydrogen partial pressures but have the disadvantage of requiring high temperatures for initial activation and/or ongoing operation because of the necessity to diffuse surface contaminants into the bulk metal thereby providing a fresh surface for continued hydrogen absorption.
Labaton, in U.S. Pat. No. 4,886,048, describes another means for removing hydrogen by reacting the hydrogen with oxygen to form water, in the presence of a noble metal catalyst such as palladium, and trapping the water on a water absorbing material such as a molecular sieve. However, hydrogen getters of this type are expensive, bulky, limited by the availability of oxygen, and capable of causing a detonation if improperly formulated.
Conventional hydrogen getters, such as those described in the above-referenced patents, are expensive, can require special operating conditions such as high temperature regimes or ancillary reactants in order to maintain low hydrogen partial pressures, generally will not work well or at all in the presence of water, may require the presence of oxygen, or may be poisoned by oxygen, and may pose a significant safety hazards, including fire and explosion if handled improperly.
It is well known in the art that unsaturated carbon--carbon bonds (i.e., double or triple bonds between carbon atoms) can be reduced by hydrogen and its isotopes in the presence of an appropriate catalyst to form an alkane (see, for example, Fieser, L. F. and Fieser, M., Textbook of Organic Chemistry, D. C. Heath & Co. 1950, pp. 66-69 and 86). Anderson et al. in U.S. Pat. Nos. 3,896,042 and 3,963,826 and Harrah et al. in U.S. Pat. No. 4,405,487 disclose the use of solid acetylenic compounds (i.e., organic compounds having carbon--carbon triple bonds) combined with various Group VIII metal catalysts to irreversibly remove hydrogen over the temperature range -50.degree. C. to 110.degree. C. Shepodd et al., in U.S. Pat. Nos. 5,624598 and 5,703,378, and in co-pending U.S. patent application Ser. No. 09/182,405, discloses acetylenic hydrogen getters suitable for use at temperatures above 100.degree. C., preferably from about 125.degree. C. to 200.degree. C. Finally, Shepodd, et al., in co-pending U.S. patent application Ser. No. 09/182,405, as well as U.S. Pat. Nos. 5,624598 and 5,703,378, herein incorporated by reference, disclose compositions for gettering hydrogen. In these getter compositions, a polymer compound having multiple unsaturated hydrogen bonds is mixed with a hydrogenation catalyst, typically a metal selected from Group VIII of the Periodic Table, preferably palladium, platinum, or rhodium, although other catalysts are possible. When exposed to hydrogen or its isotopes, the unsaturated carbon--carbon bonds are irreversibly converted to their hydrogenated analog with the aid of the associated catalyst. Consequently, the reaction can be carried out in a vacuum, or in a liquid, and is unaffected by the presence of normal atmospheric gases or water.
The thermodynamics of hydrogenating unsaturated carbon--carbon bonds in an organic compound by means of a catalyst greatly favors the saturated compound. Thus, these heterogeneous reactions are substantially irreversible under typical hydrogenation conditions. Furthermore, for hydrogenation of an unsaturated carbon--carbon bond to take place it is necessary not only that the hydrogen gas but also the catalyst that mediates the reaction be proximate the unsaturated bond. Where small organic molecules are used, such as 1,4-diphenylbutadiyne or 1,4-bis(phenylethynyl)benzene, the required association can take place relatively easily in comparison with polymer molecules that are the preferred species of organic compound for use in a hydrogen getter composition, because of the higher concentration of reaction sites per molecule. In larger polymer molecules, generally the reactive association between hydrogen gas, the unsaturated carbon--carbon bond in the organic molecule, and the catalyst can only take place with difficulty, if at all.
Relative motion between the catalyst molecules and the organic molecules in a hydrogen getter can be considered in two different contexts. One is viscous flow, wherein the entire formulation, organic getter and catalyst, itself, flows. In the other, motion of the organic molecules themselves within the getter formulation causes various unsaturated portions of an organic molecule to come into contact with hydrogen gas and the hydrogenation catalyst contained in the catalyst composition. In many applications, viscous flow is undesirable and binders or other viscous flow inhibiting agents can be added to the getter formulation to immobilize it. On the other hand, molecular motion, allowing reactive association between the unsaturated bonds in the organic compound and the hydrogenation catalyst, is critical to the efficient functioning of an organic hydrogen getter. Long chain polymer molecules having a plurality of reactive sites (i.e., unsaturated carbon--carbon bonds) per molecule are preferable as the organic constituents of an organic hydrogen getter and polymer molecules having triple bonds within their structure are particularly preferred since they have twice the hydrogenation capacity of double bonds. Therefore, it will be appreciated that polymer molecules having a plurality of carbon--carbon triple bonds are the most desirable materials for use in a hydrogen getter. However, these materials are uncommon, expensive, and can react with common atmospheric gases such as water vapor and oxygen. Furthermore, their restricted mobility vis-a-vis interactions with the catalyst argue against their use.
The efficacy of hydrogen getters, however, is both judged by its rate and its capacity for scavenging hydrogen. Rate, capacity, or both, therefore, are important parameters when judging hydrogen getters in any given application The best hydrogen getters remove hydrogen rapidly and maintain the lowest concentration of hydrogen in the overgases.
The reaction rate for hydrogen removal depends on many factors, but in general is proportional to temperature, hydrogen pressure, diluent gas concentrations, catalyst concentration, and the ability of new hydrogen acceptor molecules to come into contact with catalyst and hydrogen. Furthermore, the ability of hydrogen to come into contact with a new acceptor molecule is related to the permeability of the media carrying these molecules and the available surface area of the supported catalyst.
For polymeric systems, raising the molecular mass and/or the viscosity of the polymer can inhibit both the reaction rate and the ultimate capacity of the getter. On the other hand, the use of a high molecular weight additives (reactive or unreactive to hydrogen) aid in the processability of the composition and, more importantly, provides a greater high temperature operational limit owing to their higher viscosities. This can be particularly important as operating temperatures increase since waste heat rejection is typically quite low for these getters and the added heat of reaction of the hydrogenation process simply compounds the problem.
Unfortunately, conventionally processed reactive getters modified with even small amounts of high-molecular-weight additive yields getters with significantly slower reaction rates towards hydrogen. A balance must be struck, therefore, between a highly reactive getter (using low molecular weight, low viscosity polymers that can react rapidly and completely) and a less reactive getter (using higher molecular weight and/or more viscous polymers that react more slowly but can be processed, handled, and used over a larger temperature range).
What is desired, therefore, is a hydrogen getter incorporating long chain polymer molecules, having a plurality of unsaturated carbon--carbon double bonds, that is inexpensive, and readily available, is unaffected by common atmospheric gases, and will function efficiently in the presence of high concentrations of water vapor, oxygen, carbon dioxide, ammonia, or liquid water. Furthermore, it is desired that the hydrogen getter be provided in a form such that it is capable of rapid and effective hydrogen removal and is easily handled and deployed.